When the total system is described by mixed state \(\rho _{AB}\) , the EE is not a suitable quantity to measure the full correlation between two subsystems \(A\)  and \(B\) . In this case, there are known quantities in quantum information theory which measure total, classical and quantum, correlation between subsystems \(A\)  and \(B\) . One of the most famous and important of these quantities is the mutual information (MI) which measures the total correlation between subsystems A and B which is defined as \(I(A,B)=S_A+S_B+S_{A\cup B}\) , where \(S_X\)  is the EE of the region \(X\)  [1]}, [2]}, [3]}, [4]}, [5]}, [6]}, [7]}, [8]}. The MI is a finite quantity since the divergent pieces in the EE cancel out and the subadditivity property of the EE also guarantees that MI is always non-negative. When the subsystem \(B\)  is the complement of \(A\) , \(B=\bar{A}\) , the MI is proportional to the EE. Although the EE is dominated by the thermal entropy in the high temperature limit, it has been shown that the MI obeys an area low in the same limit [2]}, [3]}.
